With the rapid development of internet television and smart television, video content source of digital television is increasingly diversified, wherein online video has become one of the major video contents being watched on television. Due to the limitations of certain factors such as bandwidth and media storage, compression and decompression technology are applied to online video for removing redundant information so as to achieve a low bit-rate transmission, compression standard of which includes video coding and decoding standard of block-based discrete cosine transform (Block-based Discrete Cosine Transform, BDCT) such as MPEG-4 Visual, H.264/AVC, AVS and so on.
However, online video is affected by factors like transmission manner and code-rate control during transmission process, the content of original video image is subjected to distortion of different extents. Positive and negative DCT transformation itself is lossless to video image, however, quantization and inverse quantization during a coding process are main reasons in generating distortion during compression. The distortion caused by video compression primarily includes ringing artifacts and blocking artifact. Near edges of an image, coarse quantization applied to high-frequency information of an image would result in ringing artifacts; in a block-based coding mode, since the correlation between adjacent blocks during block coding is not considered, quantization differences between blocks leads to inconsistency between the blocks, thereby generating blocking artifacts. If such low quality video is displayed on a large-screen flat-panel television, user's visual experience will be seriously affected.
In the existing technology of ringing artifact suppression, US patent publication No. U.S. Pat. No. 7,894,685B2 discloses a method and an apparatus for reducing ringing artifact. The method classifies pixels according to information like mean absolute deviation (Mean Absolute Deviation, MAD) and sum of gradients of pixels' brightness, based on this, counts the number of pixels in flat area and at edges of strong/weak within a 8*8 coding-decoding block, finds an edge block satisfying the conditions and then conducts a Sigma filtering to non-edge flat pixels within the block so as to reduce the ringing artifact. The patent requires coding-decoding information like quantization parameter (Quantization Parameter, QP) to control filtering strength, in addition, the process method based on an 8*8 block is not well applicable for many universal international video coding standards. Therefore, when coding-decoding information like QP is unknown or block with a variable size is adopted during coding-decoding, the effect of the method will be restricted. In other technologies of ringing artifact suppression of the prior art, either image details are blurred while effectively reducing ringing artifact, or effect of ringing artifact reduction is poor while effectively protecting the detail information, i.e., it is difficult to keep a balance therebetween.
Existing method for detecting blocking artifact mainly conducts a block boundary strength detection when the location of block boundary is known. Due to the diversity of original resolution and coding-decoding standards in online video, the video might have already been processed when arriving at the display end, for example, being simply scaling or shifted to any direction. In this case, the result of detecting the block strength at a known block location is certainly inaccurate.
In conclusion, it is necessary to propose a method having an improved robustness, which effectively reduces ringing artifact and blocking artifact while protecting important original information such as edge information and texture information.